Understanding the molecular basis for a wide-range of biological processes has been a major success of the direct application of chemical techniques to biological problems. More recently, the awareness of the importance of the mechanical properties of living cells and their mechanical interaction with the extra-cellular environment is rapidly growing in the biological community. It is now clear that cells partake in a sensitive force balance, maintenance of which is a key factor to normal cellular function. For example, it has been observed that cellular proliferation and survival is optimal when culture substrates match their natural in-vivo stiffness. In the area of stem cell biology, researchers have discovered strong correlations between extra-cellular matrix (ECM) stiffness and stem cell lineage. For many cells types, including tumor cells, cells have been shown to generate compensatory forces in response to external loads or matrix stiffness increases in a process called mechanoreciprocity.
Such evidence supports hypotheses in which upsetting the force balance can have a significant impact on cellular behavior in cancer related processes. For example, pressure was shown to upregulate the Src-PI3K-FAK-Akt signaling pathway, which increases cell membrane-ECM adhesion in tumor cells. Tumor progression in vivo and in 3D mammary epithelial cell (MEC) culture correlated with increased ECM crosslinking and stiffness.
For instance, the plasma membrane is mechanically coupled to the inside of cell, where it interacts with the dynamic actin cytoskeleton, and not surprisingly, it has been shown that cytoskeleton rearrangement can have profound effects on plasma membrane deformability, tension, and fluidity. Also, the plasma membrane is mechanically coupled to the ECM on the outside of the cell through receptors including the integrin family transmembrane adhesion molecules. Membrane fluidity and curvature plays a direct role in facilitating clustering of these proteins, which initiates mechano-signaling to promote differentiation, proliferation and invasion. Additionally, lipid rafts, a component of the plasma membrane that is associated with a mechanical response, can modulate local membrane stiffness. These examples not only involve the interface, but also rely on fundamentally active processes driving the system out of equilibrium. This represents a small subset of roles and effectors of membrane stiffness, both known and yet to be discovered. The search for new roles and effectors should be carried out in a natural 3D context.
Most methods attempt to use micron scale particles placed in the interface and measure their fluctuations. This does not really solve the problem as the particles have too much contact with the fluid on either side of the interface (being much larger than the molecular layer of interest) and they disturb the layer, changing its properties. Both of these are major disadvantages that needed to be overcome.